Output Problems DJM 900 NXS2

I have a problem with my mixer. At first, I thought my turntables were defective.
No matter which source I connect to the mixer or which channel is connected – it’s always the same problem. So it’s not the phono input, but a general issue.

After about 10 minutes, the mixer stops receiving any signals. The input level is high enough. It then seems to fall into some kind of sleep mode. I can only reactivate it by switching the top knob to Digital Input or another source, and then back to the original position. I have already disabled the sleep mode. Has anyone encountered this problem?